[SEE UPDATE BELOW: New sources now claim training mission was a "cover story"]

A group of armed Zetas, dressed in black, earlier today shot two U.S. federal agents on a Mexican highway after setting up a roadblock ambush, according to a law enforcement source with inside knowledge of the attack.

One of the agents is dead, the other is “clinging to life” in a hospital in Mexico City, the source says.

“They [the two agents] were targeted,” the law enforcer adds. “They [the assailants] knew they were agents.”

The law enforcement source who spoke with Narco News says one of the agents is from San Antonio, the other from El Paso, Texas. Both had been sent to Mexico on temporary duty to participate in training with Mexican law enforcers, the source says.

Multiple media outlets are reporting some details of the shooting, which involved two agents with U.S. Immigration and Customs Enforcement, or ICE, which is part of the Department of Homeland Security.

The incident allegedly took place along a main highway connecting Mexico City and Monterrey.

Members of the Mexican military had escorted the agents from Mexico City to Monterrey, where the agents participated in training sessions. However, the two agents, for unknown reasons, left Monterrey early, according to the source, to attend other meetings. As a result, they were not under military escort on that return trip to Mexico City this afternoon, which is when they were ambushed.

The agents were driving a black SUV with diplomatic plates. After their vehicle was stopped, with the road in front of them and behind them blocked off (a tactic typically employed by professional assassins), several armed men dressed in black approached the agents’ SUV.

The men, according to the source, are presumed to be members of Los Zetas— a paramilitary narco-trafficking organization that counts among its leadership former elite members of the Mexican military.

As the armed men walked toward the ICE agents’ SUV, they motioned the agent in the driver seat to roll down his window, according to the source, recounting the initial reports from the scene.

“He did a little, and that’s when they opened fire,” the source reports. “The windows on the SUV are dark, so it is my opinion that they wanted make sure they had the right people.”

DHS Secretary Janet Napolitano, in a prepared statement, confirmed that one agent died and another was critically wounded (shot in the arm and leg) in the attack.

“I’m deeply saddened by the news that earlier today, two U.S. Immigration and Customs Enforcement (ICE) special agents assigned to the ICE Attaché office in Mexico City were shot in the line of duty while driving between Mexico City and Monterrey by unknown assailants,” Napolitano said in a prepared statement.

Napolitano added: “U.S. law enforcement agencies are working closely with Mexican authorities who are investigating the shooting to ensure the perpetrators of this unconscionable crime are captured as quickly as possible. …”

The law enforcement source who spoke with Narco News says he is confident the information he has so far on the attack is on the money, but adds that details are still coming in on the shooting and its aftermath.

“The situation remains very fluid,” the source adds.

Stay tuned ....

UPDATE 11:25 p.m. 2/15/11

New information has just come in concerning the background and alleged mission of the two ICE agents who were shot in Mexico earlier today.

Additional law enforcement sources have weighed in on the story and contend that the two ICE agents were on what one described as an under-the-radar mission that involved transporting a box containing an unknown payload, though it is suspected that the box contained weapons. The two agents were expecting to meet up with another group of agents to exchange the box, the law enforcers contend.

“So they were either tailed or there’s a mole somewhere who tipped them [the Zetas] off to the operation,” a law enforcement source told Narco News.

That same source described the box transfer operation as being very unusual and possibly part of some kind of “black op” — meaning it might have dovetailed with an intelligence operation.

The new information also contradicts the original information reported by Narco News in some areas. The law enforcement sources contend that is because a cover story had to be created and it is now starting to intersect with the real story.

For example, the information now coming in, which is from several law enforcement sources, indicates the agents were not involved in any type of training operations, but rather that was a cover for their trip. In addition, in the latest version of events, the agents who were shot were driving north from Mexico City heading toward Monterrey when they were ambushed by the Zetas. (A law enforcement source told Narco News originally that the two agents were heading south from Monterrey toward Mexico City.)

Also, both agents were, in fact, permanently assigned to the ICE attaché office in Mexico City and not on temporary assignment, as originally reported.

One of the agents did work in El Paso for a time in the mid-2000s, but the other agent grew up in Brownsville and launched his ICE career in Laredo, the sources contend. Neither of the agents was from San Antonio, as originally reported, according to the new information coming into Narco News.

The driver of the SUV, the sources add, survived the attack and was initially medevaced to a hospital in Mexico City but has since been transferred to an unknown hospital in the Houston area, the sources say. The passenger, however, died of gunshot wounds to the chest “and other unknown areas,” the law enforcement sources say.

“The two [agents] were going to Monterrey from Mexico City to transfer a box to agents in Monterrey [who were to meet them halfway on the trip]. It is unknown what was in that box, but it’s possible it was guns,” a law enforcement source says.

The details of the ambush are essentially the same as originally reported. However, the new information coming into Narco News includes a few more details, such as the fact that the windows on the SUV were bullet proof.

“When the agent on the passenger side rolled down his window, the unknown male [the alleged Zeta] pulled a gun and started firing through the crack in the window, hitting the passenger in the chest, and other areas, while the driver was hit in the arm and leg,” a law enforcement source says. “The driver took off immediately [somehow getting around the roadblock]. It’s really a miracle they weren’t chased down and killed then and there.”

As further updates come in, the story will be advanced, and corrected where necessary. Bear with the twists and turns in the story, kind readers. You are getting a street-level view of the scene, which is always a bit foggy as agents on the ground continue to investigate the crime, working through the conflicting information to get at the truth.